1. Field of the Invention
The present invention relates to crystal evaluation apparatus and crystal evaluation method for use in the manufacture of semiconductor devices.
2. Description of the Related Art
Ion implantation, one of techniques of introducing an impurity in the industry of semiconductor devices manufacturing, is widely used thanks of accurate control of the dose of the impurity. The ion implantation technique is one by which an impurity element is ionized and a desired ion seed is selected with mass spectrograph and accelerated with a desired acceleration energy to implant the impurity on a semiconductor substrate into a desired amount of dose at the desired acceleration energy, thereby forming an impurity layer of a desired conductive type and conductivity through heat treatments at a desired temperature and time. Amorphous layers and defective layers developed during ion implantation (particularly called "primary implantation defects") are cured through the heat treatments. It is, however, difficult to restore perfect crystal form through the heat treatments so that turbulence in crystal structure (particularly called "secondary implantation defects") remain over the semiconductor substrate even after the heat treatments. One way of evaluating such secondary implantation defects is to utilize selective wet etching to emphasize the defects and observe the same with an optical microscope or scanning electron microscope as a device of observation and another way is to observe the defects directly with a transmitting electron microscope (TEM). Further, another way was developed by which a scanning tunneling microscope (STM), one of scanning probe microscopes, is used to observe the defects at surfaces. One of such an example is I. H. Wilson et al: Physical Review B Vol. 38, No. 12 pp. 8444-8450 (1988-II) which discusses ways of observing the tracks of primary defects through which ion implantation atoms pass at a surface of a silicon substrate, after removal of an oxide layer in a silicon/oxide structure developed by ion implantation.
The following discusses methods of measuring the dopant concentration distribution over the impurity layer. Accelerated ions are caused to collide with measuring specimen and cut a surface of the measuring specimen. A secondary ion mass spectrography (SIMS) is used wherein atoms and molecules sputtering from the surface of the specimen are ionized (secondary ions) to determine the secondary ions with the mass spectrography. Recently, another way of evaluating (surface analysis) an impurity introduction layer was reported by T. Takigami et al: Applied Physics Letters, Vol. 58, pp. 2288-2290; Taking advantage of the property that the speed of wet etching is different depending upon the dose of the impurity introduced, this method transforms variation in the impurity concentration into variation in three-dimensional topographic form which in turn is measured with a scanning tunneling microscope having a space resolution on the order of nano meter (10.sup.-9 m).
Of the defect evaluation methods as discussed above, the method of utilizing wet etching would not be able to know directly the microscopic structures of the defects themselves, because the magnitude of the defects is too emphasized. The crystal defect evaluation method using the transmitting electron microscope would not observe the defects present only at the surface, because it observes the defects through a region of the specimen having a thickness of 30 to 300 nm. Moreover, the transmitting electron microscope suffers from variations from the original crystalline form because physical damages such as argon sputter are done in preparing the specimen. Although the STM evaluation method may achieve observation with a high resolution on the order of atoms, the measuring surface is limited to a surface of the specimen and the specimen should be an electrically conductive material. Another evaluation method using an atomic force microscope (AFM) does not need electrically conductive specimen but may achieve nothing more than surface evaluation like the STM method.
With the advance in technology toward finer-scale silicon devices, precise measurement of the impurity concentration (surface analysis) on the order of nano meter is needed. For instance, when the impurity concentration distribution over the source and drain regions (an impurity introduction layer) of a MOS type transistor is measured, the measuring area is less than 300 nm.sup.2 and the impurity concentration ranges from 10.sup.16 to 10.sup.20 cm.sup.-3. The conventional SIMS method known as impurity analysis method may make surface analysis. However, this method has the disadvantage that the diameter of ion beams may not be reduced as much as possible due to the need to enhance measuring sensitivity and the space resolution is thus as low as about 500 nm. When the scanning tunneling microscope and wet etching are used in combination, measurement with high resolution on the order of nano meter is possible but the measuring specimen should be electrically conductive. Another problem with such combination is that etching speed may vary with even slight variation in the mixing ratio of a wet etchant and temperature difference to impair reproducibility, because the impurity concentration distribution is transformed into a three-dimensional topographic form through the use of wet etching.
With the above discussed problems in mind, the present invention is to provide crystal defect evaluation apparatus which is capable of observing the shape of defects directly with a high resolution on the order of atoms without affecting crystal form, whether measuring specimen are electrically conductive or whatever conductivity the specimen has.
Another object of the present invention is to provide an impurity distribution measuring method having a high resolution on the order of nano meter and excellent reproducibility.